1. Field of the Invention
The present invention relates to a method and an apparatus for automatically assembling veneer sheets into a continuous plywood having desired number of layers and, more particularly, to a method and an apparatus for laminating the veneer sheets in a stepped manner so that joints in each layer or ply are longitudinally spaced from the joints in the vertically adjacent plies at a predetermined distance.
2. Description of the Prior Art
The plywood having the stepped lamination is often called as a laminated veneer lumber (LVL) or laminated veneer board (LVB). The longitudinal intervals at which the joints in one ply are spaced from the joints in the adjacent plies must be at least more than eight a to ten times as large as a thickness of the veneer sheet and are preferably more than twenty times as large as the thickness. If the interval is too short, the plywood has only an insufficient strength and tends to fracture along the joints. The joints are usually visible in the side surfaces of plywood and, therefore, arrangement of the joints at regular intervals improves an external appearance of the plywood and generally increases value thereof in the market.
The plywood of this kind has conventionally been manufactured by first forming each ply by joining the veneer sheets of a uniform size together along the longitudinal direction. Butt joint may be used in which vertical edges of the sheets are joined, but it is preferable to use a scarf joint in which the edges to be joined are beveled in the opposite directions. The plies are then laminated one on another in a stepped manner. This conventional method is however disadvantageous from a viewpoint of efficiency because of the separate two steps, i.e. joining and laminating steps.
Japanese Patent Publication No. 54-14641 discloses an apparatus for laminating the veneer sheets while simultaneously elongating the plies by adding one sheet to the rear end of each ply in sequence. The apparatus includes a roller conveyor disposed within a housing having a lower open end, the housing being maintained in sub-atmospheric pressure so that the veneer sheets coated with glue can be fed along the lower surface of the conveyor. A plurality of abutments are provided at longitudinally regular intervals in the forward end portion of conveyor and are selectively movable to project into the path for stopping the veneer sheets. When the veneer sheet is stopped by one of the abutments the rollers of the conveyor are lowered away from the housing, whereby the veneer sheet is released from the rollers and falls down to an assembling conveyor that extends just below the roller conveyor. The assembling conveyor is adapted to be driven intermittently for forwarding the veneer sheets a distance equal to the longitudinal length thereof by one operation. Thus, by moving the abutments in sequence, the veneer sheets are stopped at different positions so that they are step-laminated and added to the rear ends of the respective plies.
A similar arrangement is also disclosed in PCT International Application as published under No. WO 83/02744. An apparatus therein has a stopper movable along the longitudinal direction so as to stop the veneer sheets at desired positions. For each veneer sheet, the stopper is adapted to move to a position just above the rear edge of the ply to which the veneer sheet is to be added. It is described that the veneer sheet thus stopped is then guided to the ply.
The veneer sheets, however, tend to be displaced both in the longitudinal and transverse directions due to inertia force and frictional resistance when they are transferred or fall down. The longitudinal displacement causes an overlap of or a gap between the edges to be joined, while the transverse displacement prevents the side surfaces of the plywood from becoming smooth and flat. Furthermore, the veneer sheets supplied from a mass thereof are not always centered along the longitudinal axis of the feed conveyor and, therefore, the veneer sheets when stopped often deviate from a predetermined position in the transverse direction, which also results in uneven side surfaces of the plywood. Thus, the known devices are still unsatisfactory.
Accordingly, an object of the present invention is to provide a method and an apparatus for assembling veneer sheets into a plywood, which enables to simultaneously connect and laminate in a step manner the veneer sheets precisely without causing any overlaps and gaps in the joints and any uneven portions in the side surfaces.
Another object of the invention is to provide a method and an apparatus which permits automatic manufacture of a plywood without manual operations for adjusting positions of the veneer sheets.
A further object of the invention is to provide a method and an apparatus which enables to transport and place the veneer sheets to precise positions for assembly.